(1) Field of the Invention
The present invention relates to an optical semiconductor device and an infrared data communication apparatus eliminating a direct current component and a low frequency component included in a light signal, and amplifying a high frequency component only.
(2) Description of the Related Art
Infrared data communication apparatuses for communicating data, using an infrared light signal between: mobile communications devices; or a personal computer and a peripheral device conform to the IrDA (Infrared Data Association) standard. FIG. 1 exemplifies an optical semiconductor device in the infrared data communication apparatuses.
For example, the optical semiconductor device in the drawing receives, with a photodiode 401 a light signal, and photoelectrically converts the received light signal. Here, the light signal includes a direct current component on which an alternating current component is superimposed. The photoelectrically converted current signal is converted into a voltage by a conversion resistor 404 included in a current-voltage conversion circuit 410, Then, a differentiating circuit 409 including a capacitor 407 and a resistor 408 negates the direct current component in the voltage, and extracts the alternative current only. Thus, the alternating current component is outputted as an output voltage Vout. An optical semiconductor device in FIG. 1 is an alternating current amplifier configured above. It is to be notified that NPN transistors 402 and 403, and resistors 405 and 406 provide the photodiode 401 a bias voltage, and amplify a photoelectric current outputted from the photodiode 401, as well.
In order to increase sensitivity of the above described alternating current amplifier, typically, a resistance value of the conversion resistor 404 is enlarged. A voltage fluctuation range (voltage swing) of the conversion resistor 404 is, however, limited within a variation range of the output voltage Vout of the alternating current amplifier. Thus, one problem occurs when the resistance value of the conversion resistor 404 is enlarged in that the photoelectric current flowing into the photodiode 401; that is a dynamic range of the photoelectric current flowing into conversion resistor 404, becomes small.
For example, the largest photoelectric current Imax to be possibly flown into the conversion resistor 404 becomes significantly small to be expressed as:Imax=(5−2×0.7)/200 kΩ=18 μAwhere the resistance value of the conversion resistor 404 is 200 kΩ, a forward voltage VBE of the NPN transistors 402 and 403 is 0.7 V, and a direct current voltage of a direct current drive power source Vcc is 5V.
Further, when a low cut-off frequency fc in low frequency in the differentiating circuit 409 is conceived to be set low in the case where the alternating current amplifier is to be a monolithic integrated circuit, a capacitance value of the capacitor 407 in the differentiating circuit 409 needs to be enlarged. Thus, the only way to set the low cut-off frequency fc is to increase the size of the capacitor 407. This prevents the alternate current amplifier from downsizing.
For example, a transfer rate is 115.2 kbps in the SIR (Serial IrDA) of the IrDA. Thus, a capacitance value C, of the capacitor 407, to be needed in the case where the cut-off frequency fc in the differentiating circuit 409 is set to 100 KHz, is expressed as:fc=1/(2π×C×10 kΩ)=100 kHzwhere the resistance value of the resistor 408 in the differentiating circuit 409 is 10 kΩ. Accordingly, the capacitance value becomes relatively large to be C=160 pF.
As described above, the alternating current amplifier shown in FIG. 1 has problems in that: a dynamic range of a photoelectric current becomes small; and the alternating current amplifier cannot be downsized when an alternating current amplifier is to be a monolithic integrated circuit.
Thus, a structure shown in Patent Reference 1: Japanese Unexamined Patent Application Publication No. 04-32307 has been proposed. FIG. 2 is a circuit diagram of an alternating current amplifier photoelectrically converting a light signal including an alternating current component and a direct current component, and then extracting only the alternating current component from a current signal obtained through the photoelectric conversion, and amplifies the extracted alternating current component. The alternating current amplifier in the circuit diagram includes two photodiodes 501 and 502, a current mirror circuit 510, and a current-voltage conversion circuit 410. Here, the photodiodes 501 and 502 receive to convert a light signal into a current signal. The current mirror circuit 510 extracts only the alternating current component from the above current signal, using NPN transistors 511 and 512, and a capacitor 515. Here, the NPN transistors 511 and 512 are respectively connected to photodiodes 501 and 502, and the capacitor 515 is connected between a base and a collector of an NPN transistor 512. The current-voltage conversion circuit 410 has an identical structure to the current-voltage conversion circuit 410 in FIG. 1, and converts an output current I14 of the current mirror circuit 510 into a voltage.
Photoelectric currents I11 and I12 are equal since the photodiodes 501 and 502 are both connected to the current mirror circuit 510. Here, the photoelectric currents I11 and I12 are generated at both of the photodiodes 501 and 502 upon receiving the light signal. Here, the following holds:I11=I12=IDC+IAC where, for each of the photoelectric currents I11 and I12, the direct current component is IDC and the alternating current component is IAC.
The photoelectric current I11 directly flows into the NPN transistor 511. With regard to the photoelectric current I12, only the direct current component IDC in the photoelectric current I12 flows into the NPN transistor 512, since the capacitor 515 connected between the collector and the base of the NPN transistor 512 performs an integral action. Here:I13=IDC is held, where a current flowing into the NPN transistor 512 is I13. A current I14 flowing into the conversion resistor 404 is held as:I14=I12−I13=(IDC+IAC)−IDC=IAC.In other words, only the alternating current component IAC of the photoelectric current I12 equivalent to the photoelectric current I11 is flown into the conversion resistor 404. Where a resistance value of the conversion resistor 404 is set to R, an output voltage Vout is expressed as:Vout=R×IAC Here, only the alternating current component is extracted.
The largest photoelectric currents I11 and I12 to be possibly flown into the conversion resistor 404 are determined by the largest current value to be possibly flown into the current mirror circuit 510, regardless of the resistance value of the conversion resistor 404. In theory, the current mirror circuit 510 is operationally capable until a rise of a collector potential at the NPN transistor 511 nears a direct current voltage of a direct current drive power source Vcc, the rise occurring by a base current of the NPN transistor 511 flowing into a resistor 513. Considering a practical level, however, in the case where an end-to-end voltage of the resistor 513 is assumed to be raised approximately up to 0.1V, the largest value Imax of the photoelectric currents I11 and I12 is expressed as follows:Imax=(0.1V/10 kΩ)×100=1 mAwhere the resistance value of the resistor 513 is 10 kΩ, and a current amplification factor of the NPN transistor 511 is 100. Therefore, the Imax (here, 1 mA) is approximately 50 times as great as 18 μA; namely, the largest photoelectric current described in the FIG. 1, and thus, a dynamic range of an input photoelectric current can be kept significantly high even in the case where sensitivity of the photodiode is high.
Now, a capacitance of the capacitor 515 is described when the cut-off frequency fc is set to the 100 kHz described in FIG. 1, in the case where the alternating current amplifier is to be a monolithic integrated circuit. Where the cut-off frequency fc of the current mirror circuit 510 is 100 kHz, as described above, a resistance value of the resistor 514 is 10 kΩ, and the current amplification factor of the NPN transistor 512 is 100, the following expression:fc=1/(2π×C×10 kΩ)=100 kHzleads out C=1.6 μF, which is 1/100 of 160 pF described in FIG. 1. Thus, the capacitor 515 can be significantly downsized.
However, the optical semiconductor device in the above related art has following problems.
In the IrDA, the transfer rate is accelerating due to an increasing amount of data to be transferred. The FIR (Fast Infrared) requires the transfer rate of 4 Mbps, and the VFIR (Very Fast Infrared), which is faster than the FIR, requires the transfer rate of 16 Mbps. Further, the UFIR (Ultra Fast Infrared), which is currently under study, requires the transfer rate of 100 Mbps. This requires an optical semiconductor device to operate fast. The optical semiconductor device in the above related art, meanwhile, uses a current mirror circuit. Thus, a response speed of the current mirror circuit is slow in transition from a no-signal state to an operation start with a signal inputted. In other words, the conventional optical semiconductor device has a problem of not operating in a high frequency band (100 MHz, for example).
Moreover, along with the accelerating operation speed, noise generated by the optical semiconductor device increases. To be more specific, a high speed operation of the current mirror circuit from no signal conditions requires the NPN transistor, structuring the current mirror circuit, to be under an active state with a direct current always flown. Hence, noise which current mirror circuit itself generates cannot be ignored. In order to improve an S/N ratio, therefore, unfortunately, low exogenous noise ranging between the direct current and several tens of megahertz needs to be cut down, the several tens of megahertz being close to a signal band.
In order to be compatible with the transfer rate of 100 Mbps as described above: the noise ranging between the direct current and several tens of megahertz needs to be cut down, the several tens of megahertz being close to a signal band; and a high frequency component, which equals to or more than several tens of megahertz, needs to be amplified, as well.